As cellular technology transitions from 2G to 3G, size reduction and integration of components becomes more important with increasing device functionality. For example, many mobile phones are already incorporating GSM/PCS/DCS and WCDMA functionality. Low temperature co-fired ceramics (LTCC) has proven to be a useful integration technology wherein RF passive components may be embedded while active devices are conventionally surface mounted on the substrate. Due to the shrinking form factor of wireless devices, height restrictions generally impose an upper limit on the thickness of, for example, an LTCC substrate in addition to the height of surface mount components. For some RF components, such as LTCC filters, the performance (i.e., loss or rejection) may be dependent on the height. In general, the height improves the quality factor (Q) of embedded resonators, which in turn results in lower passband loss and/or greater out of band rejection.
In a conventional example, a filter may require 1.6 mm of height in order to meet specified performance goals. If the maximum module height is 1.6 mm, then either the entire LTCC substrate would be required to be 1.6 mm (which generally precludes the use of SMT components) or the LTCC substrate would need to be 1.0 mm (assuming 0.5 mm high SMT components and a lid), which generally precludes designing a filter that will meet specifications. Accordingly, a method would be desirable wherein the thickness of the ceramic substrate may be greater in an area where the filters are integrated and thinner in areas where SMT components are to be placed and/or lower Q-factor RF components may be embedded.